1. Field of the Invention
The invention relates in general to the field of engines, particularly to gasoline-type internal combustion engines, although it is also applicable to air compressors, gas, and diesel cycle engines. More specifically, the invention relates to cam systems used with internal combustion engines to vary the actuation, timing, duration, lift, and operation of valves.
2. Background and Description of the Related Art
An internal combustion engine burns fuel within one or more cylinders and converts the expansive force of combustion into a motive power able to do work. In an internal combustion engine for a vehicle (such an automobile or motorcycle), this process involves converting the combustion force into rotational force on the crankshaft which is then transferred to move the vehicle.
Each cylinder of an internal combustion engine contains a reciprocating piston. The piston is contained within the cylinder in a tight-fit sliding arrangement that permits only a linear reciprocating motion. In a typical four-stroke engine, the piston requires four movements (strokes) for each complete power cycle, each stroke lasting 180 degrees or one-half of a crankshaft revolution. The first stroke is the intake cycle, in which the piston moves downward from approximately its top dead center position. This creates a vacuum within the cylinder, and outside atmospheric pressure forces a gaseous air-fuel mixture into the cylinder. The second stroke, or compression cycle, is an upward movement of the piston from approximately bottom dead center position to compress the air-fuel mixture in the cylinder. Combustion takes place during the start of the third stroke. The air-fuel mixture is ignited, such as through a spark plug, and the expansive/explosive force of the ignited gases pushes the piston downward. This third stroke is also called the power stroke, and it is the resultant force that is transmitted to whatever workload is being driven by the engine, such as the power output drive shaft of a vehicle. The fourth stroke, or the exhaust cycle, occurs during an upward movement of the piston to force the burned gases out of the cylinder. This also prepares the cylinder for the start of a new complete cycle.
An important aspect of the four-stroke internal combustion engine is a series of valves that open and close a plurality of valve actuated fluid ports to allow the flow of fuel-air mixture into the cylinder during the intake stroke, and allow the burned gases to be removed from the cylinder during the exhaust stroke, but provide air-tight seals during the compression and combustion strokes. The timing of the opening and closing of these valves is critical to the engine's function. Each cylinder contains one or more intake valves, and one or more exhaust valves.
Generally, these valves are opened by a camshaft or camshafts containing a number of conventional camlobes. Camlobes are non-circular shapes (the most common is egg-shaped) that act on the valve causing it to move. Camlobes may transmit force directly to the valve stem, or indirectly through lifters, rocker arms, pushrods or other valve actuating components. For example, in a direct acting system the camlobe may be coupled to a valve stem by bucket tappets, or other suitable coupling members or linkages to cause the valve to open during a certain period of camshaft rotation when the shape of the camlobe causes the valve stem to move (a translational force). When the camshaft has rotated sufficiently to remove the force of the camlobe on the valve stem, valve springs are typically used to return the valve to a closed position. Alternatively, in a positive open and closing system, such as the desmodromic type system currently used in certain motorcycle applications, separate camlobes may be used both for opening or closing the individual valves.
During the exhaust stroke but before the piston reaches bottom dead center, when most of the air-fuel mixture has been burned, the exhaust valve opens and the pressure in the cylinder begins to push the exhaust gases out. The piston then begins its upward movement, forcing the remainder of the spent fuel-air mixture out. While the piston is moving upward, the exhaust valve goes through its maximum lift position and begins to close. The period a valve is open is known as the duration of the valve lift.
Moving toward the intake stroke, the intake valve begins to open before the exhaust valve is completely closed, and before the piston reaches the top dead center position. This period in which both intake and exhaust valves are open is called overlap. The timing of valve opening and closing, and amounts of lift, duration, and overlap are critical elements in design of cams, camshafts, and other valve actuating components.
One problem that has plagued the internal combustion engine is designing a cam system that provides a combination of efficiency and performance across a wide range of engine speeds. For example, at low engine speeds, where increased torque is desired, the intake valves are opened later allowing the cylinders to fill with air-fuel mixture very effectively. In this case, little or no overlap is desired, since overlap may allow unburned fuel to flow out through the exhaust port (increasing emissions) and burned exhaust gases to mingle with the intake flow. This is remedied at lower engine speeds by early exhaust closing.
Conversely, at higher engine speeds, where maximum horsepower is desired, the intake cycle begins earlier to take advantage of charge inertia and closes later with some charge reversion. On extended overlap (with a later closing exhaust) this earlier intake cycle leads to some charge loss, a portion of the air-fuel charge going out the closing exhaust port opened during the end of the combustion cycle.
The overall intake and exhaust cycles are longer with the timing occurring for earlier opening points and later closing points, though the actual effective timing is shorter due to charge loss, dilution, and reversion. The mean volume of trapped charge is greater than the efficient low engine speed timing marks. In this case, an earlier and longer timing and duration with long overlap is desired. If the intake valve is not opened earlier and closed later, a smaller volume of fuel-air mixture will be introduced into the cylinder hindering engine performance at higher engine speeds. Thus, the amounts of overlap are a critical part of the engine's performance.
When most of the exhaust gases are pushed out by the piston's upward movement during the exhaust stroke, the intake valve begins to open, overlapping with the open time of the exhaust valve. The inertia of the exhaust gases continues the flow through the exhaust port, and provides an initial draw for the start of the intake flow. Generally, because of the need to overcome inertia in the air column outside the intake port, the early portion of the intake valve opening period does not provide much flow of the air-fuel mixture. This is also true because the valve accelerates more slowly at the beginning and end of each opening and closing cycle, to reduce high impact wear on the valve and valve seat (and noise) from rapid sealing contact, all of which is an inherent design compromise with conventional camlobe systems.
When the piston passes up to top dead center and begins its downward stroke, the intake valve opens to its maximum lift allowing the greatest possible volume of the fuel-air mixture to flow into the cylinder. The dwell period of the cam rotation in which the valve remains open is also known as the duration, and is generally defined in terms of dwell degrees of crank-shaft rotation. The intake valve closes, usually slightly after reaching bottom dead center, so that cylinder pressure can be developed during the compression stroke of the piston. Here valve timing is important because the valve needs to be open long enough for a large capacity charge of fuel-air mixture to fill the cylinder, but must close soon enough, and quickly enough, to allow maximum cylinder pressure to develop through charge trapping.
As can be seen, there are several critical parts of the engine cycle affected by the design of the cam system. The amount of overlap, and the timing of valve opening and closing, are critical parts of the engine cycle, and are best varied with the rotational speed of the engine. The amounts of valve lift and duration, are also important considerations for maximizing the overall dynamic performance envelope.
In the traditional egg-shaped camlobe valve actuating system, the system has been designed for a compromise between low and high speed engine performance. Recently, there have been attempts to develop a variable valve timing system based on the redesign and adaptation of the traditional egg-shaped cam system. Typically these attempts have involved creating a system where the cam operation can be controlled by rotationally advancing and retarding the cam shaft in relation to its drive system or gear. This results in a change in the initial valve timing, since the camlobes will now rotate into their opening and closing positions at different locations during each complete cycle of the crankshaft. Advancing the camshaft does not affect lift or duration, only the initial timing of valve opening and closing relative to the crankshaft position. These systems typically have two positions--the cam shaft is either in its normal position (for low speed) or is advanced (for high speed), thus the valve timing is not truly variable except for a choice between two predetermined settings.
Another example of attempts to develop a variable valve timing system can be seen in those cam combinations that employ a plurality of stacked cam shaft lobes of varying shape. One lobe may be shaped for smooth low speed operating conditions, providing short duration and little overlap. Another lobe (or pair of lobes) may be adapted to provide long overlap and duration, and/or increased lift, at high engine speeds. The lobe which is operating on a given valve may be replaced by changing the position or configuration of multiple rocker arms through the use of control linking servo pistons. While this solution also provides two operating conditions, it is again not truly variable in that one of the two cam profiles is chosen for control and there are no in-between parameters. In addition, this solution adds the dynamic mass, weight, and rotational friction of additional rocker arms and cam lobes to the engine's valve actuating system, requiring greater valve opening and closing forces to overcome the greater friction inertias and thereby reducing overall engine efficiency and output horsepower.
Another area that has troubled cam system designers is the structural design of the valve and its ability to withstand the fatigue-stress forces induced by the valve's inertial mass and its reciprocating action. In relation to valve timing and the concurrent rate of change of velocity, the reason this is a concern is simple; in order to overcome the inertia of the air column in the intake stroke, it is desirable to have the valve reach its full open position as quickly as possible. However, the faster the valve is opened, the greater the force and stress introduced into the valve stem, throat, tip and valve keeper (connection between the valve stem/tip and the rocker arm or other force-transfer mechanism). Similarly, it may be desirable to close the valve as quickly as possible, either to optimize intake charge trapping to allow maximum compression as the piston begins its up stroke or to provide the longest possible valve overlap. Valve stresses, as well as the terminal speed and impact force of the valve as it contacts the valve seat, are then causes for additional concern, since in either case the valve has a limit to the severity of the stresses it can withstand without fatigue damage, or excessive wear. Moreover, this problem is complicated in that the valve system preferably has low dynamic mass weight.